The concept of using an ion-exchange membrane as an electrolyte separator for polymer electrolyte membrane (PEM) fuel cells was first introduced by General Electric in 1955. However, a real breakthrough in PEM fuel cell technology occurred in the mid-1960s, after DuPont introduced Nafion® membrane, a perfluorosulfonic acid (PFSA) membrane. Due to their inherent chemical, thermal, and oxidative stability, perfluorosulfonic acid membranes displaced unstable polystyrene sulfonic acid membranes. Even today, Nafion® membranes and other related perfluorosulfonic acid membranes are considered to be the state of the art for PEM fuel cell technology. Although perfluorosulfonic acid membrane structures are preferred, structural improvements are still needed to accommodate the increasing demands of fuel cell systems for specific applications. Higher performance, lower cost, greater durability, better water management, the capability to perform at higher temperatures, and flexibility in operating with a wide range of fuels are some of the challenges that need to be overcome before widespread commercial adoption of the technology can be implemented.
The membrane electrolyte in PEM fuel cells plays two critical roles: effectively separating both fuel and oxidant, preventing mixing; and transporting protons from the anode to the cathode to complete the redox reaction chemistry. This means that the membrane electrolyte should provide strong mechanical, chemical, and electrochemical stability and low gas permeability over a wide range of operating conditions. In addition, membranes are required to have high proton conductivity at low to medium temperatures (sub-zero to 90° C.). Usually, an ionic conductivity of 90-100 mS/cm is an acceptable and useful range for several applications in the portable, stationary, and mobile classifications, with membrane thicknesses ranging from 20 micron to 200 micron. Thicker membranes are currently preferred, due to the desire for less fuel crossover (in portable applications) and greater durability over 40,000 hours (in stationary applications). Transportation applications require thinner membranes to achieve higher power density with a durability of over 5000 hours. Today, PFSA membranes are the membranes of choice for PEM fuel cell systems, since their robustness, durability, and well-balanced physicochemical properties top general PEM requirements. Among the known PEM membranes (fluorinated and non-fluorinated), Nafion® membrane has been considered the benchmark material against which most results are compared.
The development of PFSA membranes has a long and rich history dating to the discovery of poly(tetrafluoroethylene) (PTFE) in 1938. Nafion® membrane was originally developed for chlor-alkali applications in early 1962, and later the functional group of the side chain was modified from a weaker acid to a stronger acid to suit fuel cell applications. As mentioned earlier, its first use in a fuel cell was in 1966 with its successful demonstration in NASA's space program.
Nafion® resin is a copolymer of TFE (tetrafluoroethylene) and vinyl ether [perfluoro (4-methyl-3,6-dioxa-7-octene-1-sulfonyl fluoride)] (PSEPVE). Its chemical structure in non-ionic form is shown in Structure 1. Upon further chemical treatment with a base, followed by an acid, the polymer is converted from a non-conducting film (—SO2F form) to a highly conducting (—SO3H form) ionomer membrane.

(m is about 3-11, depending upon various grades.)
After hydrolysis by a base and acidification by an acid, it becomes

The perfluorinated backbone (PTFE like) imparts superior chemical and thermal stability to non-fluorinated polymers. The pendant sulfonic acid (—SO3H) group imparts super acidic characteristics for facile proton transport. The acid capacity or membrane equivalent weight (EW), of the membrane is determined by the composition of the vinyl ether and can be typically produced from 0.67-1.25 milli-equivalents per gram, corresponding to 1500-800 EW. The proton conductivity of the membrane is strongly dependent on the water content in the membrane structure. The acid capacity strongly affects the water uptake; therefore, the conductivity of the final membrane, as shown in Table I (published in WO 00/52060 PCT Patent application). An EW range from 800-1100 is preferred for all the practical fuel cell applications considered today because it produces the maximum ionic conductivity.
TABLE IWater Uptake and Ionic Conductivity for Nafion ® MembranesEquivalentIonicWeight (g/eq)Water Uptake (wt %)Conductivity at 23° C. (S/cm)150013.30.0123135019.40.0253120021.00.0636110025.00.090298027.10.119383453.10.115278579.10.0791
The successful demonstration of PEM fuel cell operation with Nafion® membranes has stimulated other fluoropolymer producers to develop stable PFSA membrane structures. Asahi Kasei Corp. (formerly Asahi Chemical), Asahi Glass Co., and Dow Chemical Co. introduced Aciplex®, Flemion®, and Dow membranes, respectively. A chemically inert PTFE like backbone is common to all of these membranes; however, their side-chain structures are slightly different, as described in Table II. All of the membranes are currently available in the commercial market except the Dow membrane, which is no longer offered.
TABLE IITypical Functional Monomers of Perfluorosulfonic Acid MembranesMembrane TypeMonomer StructureNafion ®,CF2═CFO—CF2CF(CF3)—O—CF2CF2—SO2FFlemion ®Aciplex ®CF2═CFO—CF2CF(CF3)—O—CF2CF2CF2—SO2FDowCF2═CFO—CF2CF2—SO2F
Membranes based on neat PFSA ionomer have a tendency to swell undesirably when exposed to water, leading to poor handling and mechanical properties. This issue becomes more severe with thinner membrane structures, especially at elevated temperatures. In the early 1980s, reinforcement technologies were developed to improve the mechanical stability and durability of perfluorinated membranes, aiming at chlor-alkali electrolysis applications. PTFE-based woven fabrics and micro fibrils were widely used for reinforcement, for their chemical inertness and excellent compatibility with fluorinated ionomer structures. In 1995, W.L. Gore & Associates introduced the Gore-Select membrane, which is a micro-reinforced composite structure of expanded PTFE and perfluorosulfonic acid ionomer. A more robust and thinner membrane, as thin as 5 micron, was produced by this technique, allowing the fuel cell to achieve high power without sacrificing longevity and durability. Alternatively, Asahi Glass Co. introduced a thin, flat PTFE fibril-reinforced PFSA membrane with good mechanical strength and performance. A continuous-film production facility has been established that uses a newly developed process.
In addition, a large number of research groups are actively engaged in modifying existing PFSA membrane structures to further improve membrane functionality and durability while retaining the base membrane properties. Both physical and chemical modifications have been explored. These include impregnation of PFSA membranes with phosphoric and sulfuric acids; inorganic materials, namely, zirconium (hydrogen) phosphate and silica; and the development of blend composite membranes such as ionically cross-linked acid-base membranes, PFSA/polybenzimidazole, and perfluorinated composite membrane structures.
The instant invention relates to fluorochloro ionomers made from copolymerization of fluorochloro monomers, which form polymeric backbone, and comonomers consisting of ionic groups or precursors which can be converted to ionic groups such as acid groups or amines or quaternary amine groups. The said fluorochloro monomers are preferably perfluorochloroethyelene. Good examples include chlorotrifluoroethylene (CFCl═CF2) and dichlorodifluoroethylene (CFCl═CFCl) and the like. Chlorotrifluoroethylene is most preferred, because of its ease and safe of handling far superior to tetrafluoroethylene which is dangerously explosive. Furthermore, poly(chlorotrifluoroethylene) PCTFE has been demonstrated as the best barrier polymer against permeation of organics including methanol, far better than any known perfluorinated polymers. In fact, the barrier properties of PCTFE are 10 to 100 times better than those of PTFE or FEP for oxygen, CO2 and HCl. The most famous brand of such PCTFE barrier membrane has been Honeywell (now GE Plastics) Aclon® barrier membrane, wherein oxygen, moisture and organics all have the lowest crossover rate compared to any known plastic films. It is quite unexpected that a membrane electrolyte based on PCTFE backbone could exhibit superior barrier property against methanol crossover than PTFE backbone. Furthermore, PCTFE has also been demonstrated far stronger than PTFE in mechanical strength, which is also critical for control of undesirable membrane swollen with better water management. The combination of fluorine and chlorine in the polymeric backbone makes surprising improvement over PFSA ionomers and other ionomers. PCTFE maintains its useful service temperatures over a very broad temperature range (−100° C. to 200° C.). PCTFE has comparable mechanical strength and durability to Nylon and possesses excellent impact resistance at ambient and sub-ambient temperature. PCTFE also possesses low flame spread and low smoke generation characteristics.
Gas Permeability of PCTFE v. FEP (PTFE like) FilmsFilm Type (1 mm)MoistureO2N2CO2PCTFE0.1710.470.171.1FEP2.75-3.4450-7020-27110-190Moisture unit: kg/m2 · hrO2, N2, CO2: m3 · mm/m2 MPa
Direct methanol fuel cells (DMFC) offer high energy, compact power that is required to power numerous electronic devices for extended mission times. DMFC technology would benefit greatly with the advent of a PEM that maintains high proton conductivity under conditions of low humidity while also inhibiting methanol permeability. Swelling of the PEM due to high solvent uptake can compromise mechanical integrity and promote methanol diffusion through the membrane. A PEM that is a good proton conductor under conditions of low humidity would help to minimize these effects. Our PCTFE based polymer electrolytes unexpectedly have the advantage of improved internal water management and ion conductivity and reduced methanol crossover. Internal water balance within the PCTFE based polymer electrolyte membrane could enhance water management, and alleviate the need to recycle water formed at the cathode.
As used herein the acid form of an ionomer means that substantially all the ion exchange groups, e.g., —SO3H sulfonic groups, —COOH carboxylic acid groups, and phosphonic acid groups are protonated. One important parameter used to characterize ionomers is the equivalent weight. Within this application, the equivalent weight (EW) is defined to be the weight of the polymer in acid form required to neutralize one equivalent of NaOH. Lower EW means that there are more active ionic species (e.g., protons) present. If it takes more NaOH to neutralize the ionomers, there must be more active ionic species within the polymer. Because the ionic conductivity is generally proportional to the number of active ionic species in the polymer, one would therefore like to lower the EW in order to increase conductivity.
Lowering the equivalent weight has previously not been a very successful approach to making useful membranes, due to the physical weakness of tetrafluoroethylene backbone of current PSFA ionomers. As the equivalent weight goes down, the amount of water (or solvent) that the polymer absorbs goes up. The amount of water absorbed by the polymer is called the degree of hydration or hydration. It is expressed as the weight percent of water absorbed by the polymer after immersion in room temperature water for such as two hours. A higher degree of hydration is desirable up to a point because it tends to increase the ionic conductivity of the membrane. Correspondingly, lowering the degree of hydration has traditionally meant decreasing the conductivity. But there is a limit to the amount of water or solvent such that PFSA ionomer membranes can contain. If too much water is present, the film may lose much of its physical integrity, becoming gel-like with little rigidity and may completely disintegrate. In addition, depending on the polymer composition, low EW PFSA ionomers may even partially or completely dissolve in water. Furthermore, even if the films were mechanically stable, too high a hydration would tend to dilute the number of ions present for conduction, thereby lowering the ionic conductivity. Thus, there is an optimal degree of hydration that is high enough to provide the highest conductivity, while not so high that the films become mechanically unstable when hydrated.
Various approaches have been used to improve this limitation. In U.S. Pat. Nos. 5,654,109, 5,246,792, 5,981,097, 6,156,451, and 5,082,472, various forms of composite membranes were mentioned. Although each of these approaches may offer some improvement over a monolithic single layer PFSA ionomer membrane, they all involve the use of rather complex, composite, multilayer structures that are difficult and expensive to process. Our invented fluorochloro ionomers surprisingly improves the problem perhaps due to mechanically stronger fluorochloro polymeric backbone and low swelling when hydrated.
Fluoropolymer ionically conducting membranes have been utilized in many different applications. One application that has been widely used is as chlor-alkali electrolytic cell membranes for the electrolysis of sodium chloride, for example, in U.S. Pat. Nos. 4,358,545, 4,417,969, 4,478,695 and 6,156,451. Additionally, this generic class of polymers, described as fluoropolymer ionomers, has been proposed for use as coatings as described in U.S. Pat. No. 4,661,411; as wire insulation such as in WO 90/15828; as replacements for acid catalysts, primarily in organic synthesis as described in “Perfluorinated Resin sulfonic Acid (Nafion-H®) Catalysis in Synthesis”, by Olah, G. A., Iyer P. S, and Surya P. G. K., in Journal: Synthesis (Stuttgart), 1986 (7) 513 531, and in “Perfluorinated Resin sulfonic acid (Nafion-H) Catalysis in Organic Synthesis” by Yamato, T., in Yuki Gosei Kagaku Kyokaishi/Journal of Synthetic Organic Chemistry, volume 53, number 6, June 1995, p 487 499; as a membrane for water electrolysis as described in Yen, R. S., McBreen, J., Kissel, G., Kulesa, F. and Srinivasan, S. in the Journal of Applied Electrochemistry, volume 10, pg. 741, 1980; as a membrane for electrowinning as described, for example, in “The Use of Nation-415 Membrane in Copper Electrowinning from Chloride Solution” by Raudsepp, R., and Vreugde, M., in CIM Bulletin, 1982, V75, N842, P122; as a tube to continuously and very selectively dry wet gas streams (see product literature from Perma Pure, Inc., Toms River, N.J.); in metal ion recovery systems as described in product literature of Nafion® perfluorinated membrane case histories, DuPont Company, Polymer Products Department, Wilmington, Del. 19898; and as components in polymer electrolyte membrane (PEM) fuel cells. In the latter case, they can function both as the electrolyte or a component thereof, for example as described in by Bahar et. al. in U.S. Pat. Nos. 5,547,551 and 5,599,614; and/or as a component in one or both of the electrodes of the MEA. DuPont Nafion® perfluoro ionomer films have also been demonstrated as a good barrier to chemical agents (U.S. Pat. No. 4,515,761); however, it has not been commercial for protective covering application due to its extremely high cost.
When ionomers (ion conducting polymers) are used as the electrolyte in PEM fuel cells, they conduct protons from one electrode to the other. A common problem associated with such fuel cells is that contaminants such as carbon monoxide tend to poison the catalysts used in the MEA. These contaminants can interfere with the flow of ions between the electrodes and thus degrade the performance of the fuel cell. One way to reduce the effect of carbon monoxide is to operate the fuel cell at an elevated temperature. This reduces the formation and/or increases the destruction rate of potential contaminants and thereby allows more efficient electrode performance. The problem with running at high temperatures, however, is that it vaporizes liquid water within the fuel cell, and in so doing, tends to reduce the degree of hydration in the membrane. As described above, decreasing the hydration lowers the ionic conductivity, thereby reducing the efficiency of ion transport through the membrane and adversely affecting fuel cell operation. In fact, at lower temperatures, in PEM fuel cells using conventional ionomers, the incoming gas streams are usually well-humidified in order to maintain a relatively high degree of hydration. Only by adding the additional water in the form of humidity in the gases can the hydration be kept high enough to allow efficient fuel cell operation for long period of time. However, as the temperature gets close to, or above, the boiling point of water this approach becomes difficult and ineffective. Thus, an ionomer with relatively low hydration and acceptably high ionic conductivity would require less ambient water to function as the electrolyte in PEM fuel cells. It could function efficiently both in lower humidity environments at lower temperatures, as well as at temperatures closer to and even potentially above the boiling point of water.
Against this background of conventional wisdom, applicant has discovered fluorochloro ionomers that have a combination of relatively high ionic conductivity and relatively low hydration, and excellent barrier against permeation of organics including methanol crossover. As a result, this invention makes possible the more effective use of solid polymer electrolyte membranes in existing applications such as those described above. Additionally, new applications heretofore not practical may become possible with this new, unique set of characteristics. The instant invention is particularly valuable as an electrolyte or component thereof, or as a component in the electrode of polymer electrolyte membrane fuel cells operating at high temperature or low humidity as well as breathable protective covering industry.